This invention relates to subscriber carrier telephone systems. More particularly, it relates to an improved single channel subscriber carrier telephone system.
Frequency multiplexed subscriber carrier telephone systems have been in existence for many years. In order to save the expense of installing a pair of conductors for each telephone subscriber, a single pair of conductors is used and a plurality of channels are created for simultaneous transmission by modulating a number of carrier frequencies with voice frequencies so that the channels do not interfere with one another. Sophisticated electronic circuitry has been devised to modulate and demodulate the carrier transmissions at the subscriber terminal and the central office terminal. An example of such sophisticated electronic circuitry is disclosed in U.S. Pat. No. 3,806,655, assigned to Carrier Telephone Corporation of America, where the object was to squeeze as many carrier channels onto a common pair of conductors.
In many instances, however, the need is to provide only one additional channel on a single pair of conductors. That type of use is known as a single channel subscriber carrier system. Since only one additional channel is needed, the electronic equipment for deriving the additional channel need not be nearly so sophisticated as the circuit described in U.S. Pat. No. 3,806,655. An example of a prior art single channel carrier system is shown in FIG. 1. Subscriber circuit terminal 10 is connected to central office circuit terminal 12 through a pair of metallic conductors 14 and 16, also referred to as the tip and ring conductors. Subscriber terminal 10 is connected to a standard telephone set through conductors 18 and 20. The central office terminal 12 is connected to the central office equipment through conductors 22 and 24. Carrier subscriber terminal 26 is connected to carrier central office 28, also through metallic conductors 14 and 16. Carrier circuits 26 and 28 include the electronics required to derive and decode the carrier signal which is applied to conductors 14 and 16. Carrier subscriber terminal 26 is connected to a telephone through conductors 30 and 32. Subscriber terminal 28 is connected to the central office through conductors 34 and 36. Isolation filters 38 and 40 are provided between the voice frequency subscriber terminal 10 and the carrier subscriber terminal 26, as well as between the voice frequency central office terminal 12 and the carrier frequency central office terminal 28 to isolate the carrier frequencies which normally will be in the range of 76 khz to 28 khz from the voice frequency central office and subscriber terminals.
Subscriber carrier terminal circuit 26 includes battery 42 to power the electronics in the subscriber carrier terminal. The battery is charged through a trickle of current supplied from the central office through central office terminal 12 and through battery charger 44. The amount of power which can be drawn from the central office to charge the battery 42 must be limited to a very small amount or that power will result in an off-hook indication to the central office or interfere with the central office's ability to accurately count dial pulse signals.
There are several other drawbacks to the circuit shown in FIG. 1. Both subscribers must be on hook for the battery 42 to charge properly. If the carrier subscriber is off hook excessively, the battery rapidly discharges, leaving the system inoperative. The loop resistance of the carrier channel is limited by the relatively low battery voltage. The battery 42 is a relatively unreliable component, requiring regular replacement. Furthermore, if either subscriber leaves his phone off hook, the battery does not charge properly or is discharged, and an unnecessary service call will result.